A new fusion hypothesis for the origin of Eukarya: better than previous ones, but probably also wrong.

I discuss here the possibility that Eukarya originated from the engulfment of a thaumarchaeon by a PCV (Planctomycetes, Verrucomicrobia, Chlamydiae) bacterium, followed by invasions of NCLDV and retroviruses. The thaumarchaeon provided both informational and operational proteins (actins, ESCRT proteins), including some essential proteins absent in other archaeal phyla (Topo IB), whereas the PVC bacterium provided phospholipids, tubulin and the membrane coat proteins required for the formation of the nucleus. Viral invasions introduced many proteins that are specific to modern Eukarya and produced an arms race that favoured the evolution of Eukarya toward increasing complexity. This scenario is the best possible fusion hypothesis that can be presently proposed. However, it still requires several ad hoc assumptions to explain the origin of the nucleus and the distribution pattern of archaeal and bacterial traits in modern Eukarya. Furthermore, it still fails to explain convincingly the origin of eukaryal viruses and the existence of three distinct lineages of ribosomes. I conclude that Eukarya and their viruses more probably evolved from a specific lineage, according to the three domains scenario originally proposed by Carl Woese.